(a) Field of the Invention
The present invention relates to cerium oxide ultrafine particles and a method for preparing the same and more particularly to single crystal cerium oxide ultrafine particles which can be used as a polishing agent for flattening and finishing the surface of, for instance, glass, quartz, silicon and tungsten materials, electroless-plated nickel/phosphorus layers or films and hardmetal materials.
The polishing agent comprising such cerium oxide ultrafine particles can be used in various fields, for instance, production of optical elements such as lenses; production of electronic materials for constituting display elements such as cathode-ray tubes and liquid crystal elements; production of parts such as photomasks for constituting electronic device-manufacturing apparatuses; production of parts for recording information such as hard disks; and production of semiconductor elements, for instance, silicon wafer-processing and planation processing employed in the course of IC-manufacturing processes.
(b) Description of the Prior Art
Cerium oxide fine particles have mainly been used as carriers for catalysts and polishing agents for polishing glass materials and must satisfy different characteristic properties depending on the application.
When using the cerium oxide fine particles as carriers for catalysts, they must have a high specific surface area, a large pore volume and a large pore diameter and these requirements should likewise be maintained as much as possible even at a high temperature. For instance, U.S. Pat. No. 5,017,352 discloses ceric oxide having a specific surface area of not less than 85.+-.5 m.sup.2 /g after firing at a temperature ranging from 350 to 450.degree. C. and preferably having a specific surface area falling within the range of from 100 to 130 m.sup.2 /g after firing at a temperature ranging from 400 to 450.degree. C.; The oxide is prepared by hydrolyzing an aqueous ceric nitrate solution in an aqueous nitric acid solution, separating precipitates formed, washing the precipitates, optionally drying them and then firing the same at a temperature ranging from 300 to 600.degree. C.
In addition, U.S. Pat. No. 4,784,026 discloses ceric oxide having a specific surface area of not less than 85.+-.5 m.sup.2 /g after firing at a temperature ranging from 350 to 500.degree. C. and preferably having a specific surface area falling within the range of from 150 to 180 m.sup.2 /g after firing at a temperature ranging from 400 to 450.degree. C.; This oxide is prepared by reacting an aqueous ceric nitrate solution with a sulfate ion-containing aqueous solution to thus precipitate basic ceric sulfate particles, separating the precipitates thus formed, washing the particles, optionally drying them and then firing the same at a temperature ranging from 300 to 500.degree. C.
Further, U.S. Pat. No. 5,035,834 discloses an intermediate for use in the preparation of the foregoing fine particulate cerium oxide as well as a method for preparing the intermediate. The intermediate is a cerium(IV) compound represented by the general formula: Ce(OH) .sub.x (NO.sub.3).sub.y .multidot.pCeO.sub.2 .multidot.nH.sub.2 O (in the formula, x is a numerical value satisfying the relation: x=4-y; y is a numerical value ranging from 0.35 to 1.5; p is not less than 0 and not more than 2.0; and n is not less than 0 and not more than about 20) and the method for preparing the intermediate comprises the steps of hydrolyzing an aqueous cerium(IV) salt solution in an aqueous acidic solution, separating the resulting precipitates and optionally subjecting them to a heat-treatment. The intermediate shows the same X-ray diffraction pattern as that observed for Ceo.sub.2, but the firing loss thereof is found to be 20%. Moreover, the intermediate turns to cerium oxide having a large specific surface area by firing.
All of the cerium oxide powdery products prepared by the foregoing methods have a very small crystalline grain size on the order of about 5 .ANG. as determined by the X-ray diffractometry. In addition, they have a large specific surface area, more specifically, a specific surface area of not less than 85.+-.5 m.sup.2 /g and generally not less than 100 m.sup.2 /g, but the particle size thereof is on the order of about 0.5 to 2 .mu.m and the particles have micropores of about 50 .ANG.
Cerium oxide has been known to be most effective as a polishing agent for polishing glass materials and has widely been employed.
To polish glass materials such as lenses, there has generally been used a cerium oxide polishing agent prepared by firing a bastnasite ore which mainly comprises cerium carbonate and then finely pulverizing the fired product. Such a cerium oxide polishing agent practically used has an average particle size ranging from 1 to 3 .mu.m and is inevitably contaminated with an uncontrollable amount of impurities since a naturally occurring ore is used as a starting material. For this reason, such a polishing agent cannot be used in the semiconductor device-manufacturing processes.
There have been known methods for preparing highly pure cerium oxide which comprise the steps of adding a salt of, for instance, carbonic acid, oxalic acid or acetic acid to an aqueous solution of, for instance, purified cerous nitrate, cerous chloride or cerous sulfate to thus form precipitates of a corresponding product (such as cerous carbonate, cerous oxalate or cerous acetate); filtering the precipitates; drying them; and then firing the dried precipitates. The oxide of Ce(III) is in general unstable and cannot actually exist in the air. For this reason, cerium oxide can actually exist only in the form of CeO.sub.2. In these production methods, acid moieties such as carbonic acid, oxalic acid or acetic acid are released from the dried precipitates during the firing step as the temperature increases to thus give cerium oxide. In this respect, holes are formed at positions from which the acid moieties are released and this leads to the formation of particles having a quite low degree of crystallization. The cerium oxide having a low degree of crystallization has high chemical reactivity and when such cerium oxide is used as a polishing agent, it suffers from various problems such as burning, formation of orange peel and adhesion on the surface of a subject to be polished. Consequently, such cerium oxide cannot be used in precision polishing. For this reason, the firing temperature used in the foregoing methods should further be increased. If the firing temperature is further increased, the foregoing holes are collapsed and the crystallinity of the particles is improved, while sintering of the particles simultaneously proceeds and this in turn leads to a gradual increase in the particle size. In this regard, fine particulate cerium oxide having an average particle size ranging from 20 to 80 nm can be obtained by finely pulverizing such sintered particles having a large particle size if disregarding the particle size distribution, and therefore, the pulverized cerium oxide can be used in the semiconductor device-manufacturing processes depending on purposes. In applications wherein the polished surface requires quite strict accuracy, the particles must be uniform in particle size. However, it is impossible to obtain particles which are uniform in particle size, by pulverization of large particles.
U.S. Pat. No. 4,786,325 proposes a method capable of making the particle size uniform, which comprises the steps of continuously supplying and simultaneously mixing an aqueous cerium nitrate solution and an aqueous ammonia solution in such a manner that the equivalent amount of ammonia is not less than that of cerium and that the pH of the solution is maintained at a level of not less than 6 to thus form precipitates; filtering off the resulting precipitates; drying them; firing the dried precipitates at a temperature ranging 600 to 1200.degree. C.; and then pulverizing the resulting oxide by using a jet mill. In this method, when cerous nitrate is employed, an aqueous hydrogen peroxide solution is added to convert it into ceric nitrate and it is an essential requirement that the reaction system comprises a solution of a salt of at least one trivalent rare earth element selected from the group consisting of lanthanides and yttrium in amount of 0.5 to 60%, in addition to ceric nitrate in amount of 40 to 99.5%. Moreover, the average particle size of the resulting oxide ranges from 0.5 to 1.7 .mu.m and therefore, the oxide cannot also be used in applications wherein the polished surface requires quite strict accuracy.
Incidentally, the particle size of fine particles has often been correlated with the specific surface area thereof, in the field of ceramics and the correlation between them can be expressed by the following equation: EQU Specific Surface Area (m.sup.2 /g)=3/r.rho.
wherein r represents the diameter ( .mu.m) of the particles and .rho. means the density (g/ml) thereof.
However, the foregoing relation does not hold for any material including a large number of fine pores in the internal parts of the body. For example, the cerium oxide prepared by the production method developed while keeping, in mind, the application of the oxide as a catalyst, naturally has a large number of fine pores in the internal parts of the body and thus has a large specific surface area and therefore, the particles have a true particle size on the order of 1 .mu.m even if they have a corresponding particle size of not more than 5 nm, which is calculated from the specific surface area according to the equation defined above.
The required particle size of polishing agents may vary depending on various applications, but the higher the desired accuracy for finished surface (surface accuracy) after polishing, the smaller the particle size of the polishing agents. For instance, the polishing agent used in the semiconductor device-manufacturing processes must have a particle size ranging from 10 to 80 nm and should be uniform in the particle size. More specifically, when interlayer insulating films are polished in a semiconductor device-manufacturing process, the surface accuracy achieved after the polishing should be about 5 .ANG. as expressed in terms of the average surface roughness and the polishing agent should have a particle size of not more than 80 nm in order to fulfill the foregoing requirement. In addition, the polishing rate tends to decrease in proportion to the reduction of the particle size of the polishing agent used. For this reason, if the particle size is less than 10 nm, cerium oxide loses its advantage such that cerium oxide is superior in the polishing rate to the colloidal silica. Moreover, the polishing agent should be uniform in the particle size as much as possible in order to achieve a high degree of flatness. Accordingly, the polishing agent to be used in the semiconductor device-manufacturing processes must not only have an average particle size ranging from 10 to 80 nm, but also be uniform in the particle size.
Moreover, the polishing agent particles are preferably uniform in shape in order to ensure a desired degree of flatness. In this regard, the particles are approximately uniform in shape if they are formed from single crystals and such a polishing agent permits the achievement of an extremely high degree of flatness.
It has been known that cerium oxide can ensure the fastest polishing rate in polishing processing of silicon oxides such as quartz substrate. In addition, interlayer insulating films are formed from silicon oxide in many cases and therefore, cerium oxide would be most desirably used as a polishing agent for interlayer insulating films in order to achieve the fastest polishing rate. However, the polishing agent used for processing interlayer insulating films must in general satisfy highly strict requirements for the degree of flatness and for the surface accuracy achieved after the polishing. At present, colloidal silica is the only polishing agent whose particle size ranges from 10 to 80 nm and which has a narrow particle size distribution and it has accordingly been used in the polishing processing of interlayer insulating films, but the colloidal silica does not have a desired high polishing rate and cannot provide a sufficiently high operating efficiency. For this reason, there has been an intense desire for the development of cerium oxide whose particle size ranges from 10 to 80 nm and which has a narrow particle size distribution.